If you are running a chopper then just check on the top of the straw walkers. The out the back means off the cylinder. Do you have the low speed corn drives on this machine??? There are three different drive pulley/belt combinations used on a Gleaner "F" combine.
1) High speed (small pulley) and shorter belt. Allows up to 1000-1200 cylinder RPM
2) Mid range ( medium size pulley) Called the universal pulley because Gleaner brought it out when guys complained about have to change pulley for corn and soybean to oats and wheat. IF this is the one you have then it will not go slow enough in some corn. It only goes like 450-800 rpm. Really does not work the best on corn or wheat.
3) Corn drive pulley (Larger pulley and long belt) Drives cylinder 250 -600 RPM.
Also you should be able to open the cylinder up and THEN drive fast. If you are crowding it with the cylinder set too narrow that you will just make it grind the corn worse. Open it up until you start to see some whole rows of kernels on the cobs. Then really speed up and see if the rows of kernels disappear. If not narrow it up some and keep going fast.
Also have you checked to see if the cylinder bars are worn?? Worn out cylinder bars will make it shell funny. Also check the cross level on the concave. I have seen them off side to side and that will make them grind it on the one side and let kernels through the other.
You do have cylinder bar filler plates in?? Just checking. I had a guy on here a few years ago having all kinds of trouble and he did not have them in. Makes it impossible to run a good corn sample. I am sure you do but just wanted to ask.
Keep playing with the setting. Usually a Gleaner can be made to have a real nice grain sample. It just take some adjusting and switching of things.